1. Field of the Invention
The present invention relates generally to a noise reduction apparatus for a motion video signal, and more particularly to a noise reduction apparatus that controls the degree of noise reduction in accordance with a value (brightness) of a motion video signal.
2. Description of the Related Art
In recent years, motion video processing apparatuses have remarkably been developed and widely used, so there is a demand for high reliability. An example of such motion video processing apparatuses is a motion video noise reduction apparatus. A frame recursive noise reduction apparatus, for instance, is known as the motion video noise reduction apparatus.
In the frame recursive noise reduction apparatus, an output image that precedes by 1 frame (i.e. 1-frame-preceding output image) and a current input image are subjected to subtraction, thereby obtaining an inter-frame difference. The inter-frame difference is multiplied by a certain coefficient, and the resultant is added/subtracted to/from the current input image. The coefficient is calculated on the basis of motion detection information that is obtained from the inter-frame difference. As the motion is greater, the coefficient is set at a smaller value. Thereby, afterimage is reduced.
Patent Document 1 (Jpn. Pat. Appln. KOKAI Publication No. 6-62283) and Patent Document 2 (Jpn. Pat. Appln. KOKAI Publication No. 3-240393) disclose noise reduction apparatuses wherein motion of a real motion video component that includes no noise is also detected, and if the motion of the real motion video component is detected, a noise reduction process is temporarily suppressed or suspended. Thereby, afterimage, which is a side effect of the noise reduction process, is prevented from occurring on the screen.
In the prior-art noise reduction apparatus, there is the following problem. Noise is conspicuous, in particular, on a dark part of an image. Thus, if the dark part is subjected to a high degree of noise reduction in the noise reduction process in accordance with a difference signal, afterimage becomes conspicuous, in particular, on a light part of the image on the screen, which is a side effect of the noise reduction process. On the other hand, there is such a technical background that noise on a light part of an image is not conspicuous even if it is not removed.
Hence, it is effective to execute a noise reduction process in accordance with the brightness of motion video, for example, by increasing the degree of noise reduction for a dark part of an image and decreasing the degree of noise reduction or suspending noise reduction for a light part of the image. However, in the above-described prior-art apparatus, there is problem that noise reduction cannot be executed in consideration of the brightness (luminance, etc.) of an image at each point in time.